Wet combustion of oxidizable materials suspended and dissolved in supercritical fluid such as water is known to result in an extraordinary level of oxidation at the molecular and atomic level. Because of the ability of supercritical water to dissolve straight chain hydrocarbons, ring hydrocarbons and oxygen into solution, the desired wet combustion reaction in supercritical water is more rapid and more complete than a wet combustion reaction of heterogeneous mixtures of the same materials suspended in subcritical water. In fact, by using supercritical water in the wet combustion process, the degree of oxidation of organic materials can be so complete that organic materials are undetectable in the resultant fluid. The complete oxidation of organic materials to the undetectable limits of technology makes this process extremely valuable in the treatment of water contaminated with oxidizable materials.
However, unlike subcritical water, supercritical water will not retain materials such as minerals and salts in solution. Hence, any remaining minerals and salts will eventually precipitate from the solution. The resultant precipitate is notoriously adhesive, and therefore, tends to accumulate in the containment vessel, typically to the walls of the containment vessel or at any restrictive outlets of the containment vessel. This accumulation interrupts the desired continuous use of the apparatus and method and in the extreme, terminates the process altogether.
This problem with the adhesive, precipitated materials is further exasperated and complicated when the apparatus employed for supercritical wet oxidation uses piston stroke pumps and one-way valves for pressurization of the heterogeneous materials. That is, for example, pressurization of the heterogeneous materials may cause the one-way valves not to seat when using piston stroke pumps. Hypodermic pumps may clean the sides of the apparatus and may remove suspended solids in a superior manner, but cleaning applications employing those pumps is limited to suspended solids less than 100 microns in size. Moreover, the problem of pressure containment still exists when the stroke is reversed.
Accordingly, attempts have been made to develop methods and apparatus for supercritical wet oxidation that can control this problem of suspended solids pressurization. To that end, U.S. Pat. No. 4,594,164 first attempted to solve the problem by the use of a fluid film of subcritical water to pass between the supercritical water and the containment vessel. However, there was no control over the thickness of the fluid film, and no provision was made for the fluid film to be “clean” or free of dissolved or suspended solids. Due to these shortcomings, the process was restricted to “near” supercritical conditions as the subcritical fluid film would otherwise be quickly brought to supercritical conditions resulting in the adverse precipitations.
Later patents, such as U.S. Pat. Nos. 4,543,190 and 4,292,953 alleged benefits of supercritical wet oxidation, but neither patent made any attempt to control the precipitation of adhesive solids or salts resulting in applications limited to process streams having a flow capacity of less than 1 gallon per minute. Moreover, the deposition of adhesive solids required frequent replacement of containment vessel components.
Further, precipitated materials tend to be severely corrosive to the containment vessel as is obvious when the precipitated salt is sodium chloride, and less obvious when the precipitant is a metal such as arsenic. The corrosive tendencies tend to be so severe as to require the replacement of the containment vessel after each use, or to make the vessel out of gold, which is not practical.
Thus the problem of adequate control over the precipitation of adhesive and corrosive solids that would otherwise be dissolved or suspended in subcritical fluids, but are not in supercritical fluids, remains unsolved, as does the problem of pressurization of water with suspended solids larger than 100 microns. The fundamental need in waste water remediation using supercritical wet oxidation lies entirely within the arena of those situations for which the existing art has not resolved these problems.